The food market for the large-scale retail trade (supermarket chains and similar) requires increasingly sophisticated packaging materials, especially to increase the storability of the food and the aesthetic quality of the packages, for example in terms of container transparency. One of the technical requirements of the food distribution market is for packages consisting of a tray, or container, and a top film or cover that offer sufficient resistance to the entry of oxygen inside the tray to preserve the quality and flavour of the product contained until the use-by date indicated on the package. This parameter is defined shelf life.
A known package commonly used on the market, illustrated schematically in FIG. 1, consists of a multilayer container 10 having a total thickness of 330 microns, comprising a PET (single or three-layer) base 11 having a thickness of 280 microns, on which a PE/EVOH/PE three-layer lamination film is applied, having an overall thickness of 50 microns (22 microns+6 microns+22 microns). In some cases the PE/EVOH/PE 50 micron lamination film is replaced by a PE/PA/PE film (21 microns+8 microns+21 microns), where PA is a polyamide.
Other packages feature a coating with inorganic base (e.g. silicon oxides) deposited on the PET base via plasma processes and/or by solvent, or with an organic base (PVDC) deposited on the PET base via solvent lacquering processes, to provide adequate barrier properties which are not guaranteed by the PET.
In these known packages the top film—in FIG. 1 the cover 15 of the container 10—has a total thickness of approximately 50 microns, and is of the PE/EVOH/PE three-layer type (22 microns+6 microns+22 microns). In some cases the 50 micron top film in PE/EVOH/PE can be replaced by a five-layer film of the PE/Ad/PA/Ad/PE type (18 microns+3 microns+8 microns+3 microns+18 microns), where Ad represents a layer of adhesive.
It is clear from examination of the above-mentioned structures that both for the container and for the top film or cover the oxygen barrier is obtained from the polymeric fraction EVOH or PA, which is known to be highly efficient in blocking the entry of oxygen into the package in order to improve preservation of the food product contained. It is also clear that the structure of the two components of the package, i.e. the container and the cover, is such that the package is closed by means of the contact between the layers of PE, which are heat-sealed PE on PE in order to guarantee good closure and sealing.
Although effective, the known package described above has numerous drawbacks.
As regards the container, the PE/EVOH/PE three-layer (or PE/Ad/PA/Ad/PE five-layer) lamination film can de-laminate from the PET base if the hot lamination process is used, and in any case the risk of delamination is a problem. Alternatively bonding processes with adhesives can be used, but these increase the number of container formulation components and often require the use of chemical solvents for deposition of the adhesive.
Furthermore, the package is not perfectly transparent due to the presence of PE, EVOH and PA and any adhesives.
However, the main drawbacks of the package concern the environmental and waste management aspects during production of the packages. Said drawbacks arise mainly from the fact that the packages comprise different plastics, i.e. they are packages produced using the multi-material approach (PET; PE, EVOH and/or PA with relative adhesives, plus any other components or additives). The multi-material nature of the package severely limits—or even makes practically impossible—effective and inexpensive recycling of both the production waste and the package itself at the end of its life, due to the presence of plastics that are incompatible with one another. By the term “recycling” we mean the possibility of actual re-use of the material, for example by melting and subsequent moulding into a new product, usually a new container and/or top film which can be re-used in the same application. This type of recycling maintains the value of the material substantially unaltered. In contrast, a recycling in which the material is used for a “poorer” application than the original application diminishes significantly the value of the material.
The limitations to recycling of multi-material packages are discussed below.
In the case of the known package described above, the application of a three-layer lamination film on the PET base of the container offers satisfactory oxygen barrier properties but it is known that the chemical/physical characteristics of the EVOH (melting point, thermal stability, refractive index etc.) are such that the EVOH cannot be treated in the production process at the same extrusion temperatures as the PET (280-290° C.), as this causes phenomena of thermal degradation, decolouring and formation of black carbonaceous pitting due to the excessively high temperature to which the EVOH polymeric fraction is exposed. Furthermore said phenomenon, combined with the presence of the PE not miscible with the PET, tends to create considerable haze in the final sheet.
The use of films containing polyamides (PA), on the other hand, entails the phenomenon well known in literature of decolouring of the polyamides when treated at temperatures above 250° C., and immiscibility of the PAs with the PET. Furthermore, adequate oxygen barrier properties require the use of relatively high quantities of PA, up to approximately 5-8% of the total of the container. Therefore in the recycling phase the retention of the viscosity of the material is generally good but strong yellowing of the PA fraction also occurs, which significantly worsens the optical quality of the resulting product, in terms of both colour and haze. Excessive overheating of the polyamide fraction can also lead to embrittlement of the sheet.
The use of inorganic-based coatings deposited on the PET base, although having high barrier performance in theory, has drawbacks relative to the low actual barrier properties, as said inorganic coatings are very fragile (they create a film of vitreous material). This means that the static and dynamic stresses to which the materials are subject during processing tend to create “cracks” on the coating layer, triggering points with poor oxygen barrier which do not guarantee the performance required in all operating conditions.
The use of organic-based coatings (PVDC) deposited on the PET base has drawbacks associated with the presence of chlorinated compounds, which are not well accepted by the market and are difficult to recycle in general, particularly together with the PET.
As regards the top film or cover, it is made of polymeric material different from PET and it therefore entails the compatibility drawbacks described above.
It should also be remembered that the production process involved in forming the container into various shapes and sizes, such as packs, trays and boxes of roughly cylindrical or roughly parallelepipedal shape, and related covers, produces considerable quantities of waste, which it would be desirable to recycle in the same application but which for the reasons explained above are not re-usable. Naturally this applies even more so to recycling and reuse of the package at the end of its life.
It is therefore possible to recycle and reuse said multi-material objects only for unsophisticated applications or in any case applications less sophisticated than the primary application, where transparency, aesthetic quality and technological properties such as oxygen barrier are not required, for example garden chairs, tables and other objects obtained by injection moulding. Otherwise said materials are disposed of in waste incinerators, resulting in wastage of raw material and high environmental impact.
U.S. Pat. No. 6,455,620 B1 describes articles having improved oxygen barrier properties intended for the packaging of foodstuffs. In particular said articles contain materials capable of capturing the oxygen (scavengers), specifically via an initiating action of exposure to the UV rays, so as to prevent penetration of the oxygen into the materials and consequent contact with the food product. The materials able to capture oxygen are polyethers, an oxidation catalyst and a photoinitiator which promotes activation of the catalysis via exposure of the product to the UV rays. Example 34 of the patent describes the preparation of a 5-layer film comprising an oxygen scavenging layer consisting of a mixture of PET, a polyester-polyether copolymer, a photoinitiator and an oxidation catalyst consisting of an organic cobalt compound. The film forming the scavenger layer contains 500 ppm of Co and is not oriented. It is bonded by means of layers of adhesive to bioriented PET film (Mylar®). The final multilayer film consists of the following: Mylar®/adhesive/scavenger film/adhesive/Mylar®. The central layer represents 2/14 of the total thickness, hence the final content of Co in the multilayer film is 500/7, i.e. 71.5 ppm. The rest of the multilayer film consists of 6/14 bioriented PET film and a further 6/14 adhesive layers. There is therefore a considerable number of adhesive layers which make the product difficult to recycle and hazy. Furthermore the multilayer film is produced by means of a process of pressure lamination downstream of the extrusion process, which makes it non-advantageous in production terms.
WO 2005/023530 A1 describes a method for producing oriented single-layer PET bottles with high barrier effect and high transparency. The material forming the single layer is a mixture of PET, polyamide (MXD6), cobalt salt and ionic compatibilizer, the latter consisting of a copolyester containing a sulphonated group salified with a metal. The mixture comprises 1 to 10% by weight of polyamide, 0.1 to 2 mole % of ionic compatibilizer and 20 to 500 ppm of cobalt. Multilayer or unoriented (amorphous) articles are not described. The oxygen transmission data given in the various examples illustrated relate only to oriented film, which is known to offer much better barrier properties than those of unoriented films having the same composition.
US 2003/0134966 A1 describes barrier compositions and articles made therefrom, including blends of PET/cobalt octoate/MXD6 polyamide. Such type of blend does not produce a good material when recycled in an amount above 5% due to the presence of the polyamide.
U.S. Pat. No. 5,077,111 describes recyclable multilayer plastic perform and container blown therefrom, including a blend of either PET/MXD-nylon or PET/MXD-6 nylon/Co. The same remarks apply as for US 2003/0134966 A1 above.
WO 2009/0302560 A1 describes an oxygen scavenging composition for the production of an article with low haze comprising a polyester, a copolyester-ether and an oxidation catalyst such as a cobalt salt. A single-layer oriented article such as the wall of a bottle is also described. The formulations described contain 100 ppm of cobalt and different concentrations of polyether. Multilayer or unoriented articles are not described. As regards the oxygen barrier data, the minimum oxygen permeability value is given in example 6 with 1% by weight of copolyester-ether type D. The permeability found is 0.03 cc/cm/(m2·day·atm) on an oriented article such as a bottle having a thickness of 0.25 mm. This value of 0.03 refers to a thickness of 1 cm. A film having the same composition but with thickness 200 times lower, i.e. 50 microns, would show a permeability to oxygen 200 times higher, i.e. 0.03×200=6. The description also mentions the use of thermoformed articles such as trays or cups for foodstuffs. It is known that the oxygen permeability of said unoriented articles is 4-5 times higher than that of corresponding oriented articles, at the same thickness.
The state of the art illustrated above therefore does not solve, or unsatisfactorily solves, the problem of obtaining an unoriented film made of thermoplastic material for the production of a container which can be completely recycled in the same application, has the lowest possible content of metal elements such as cobalt or other metals and at the same time offers an adequate oxygen barrier, thus making it suitable for the preservation of food.